Indocyanine green (ICG), a near-infrared (NIR) fluorescent dye, is a substance approved by the US Food and Drug Administration (FDA) for use in the examination of the lymphatic system, the heart, the liver and the vascular system. Particularly, indocyanine green is known as an excellent probe for imaging of metastatic lymph nodes and mapping of sentinel lymph nodes, which are performed for early diagnosis of breast cancer.
A sentinel lymph node is the first lymph node which is closest to a primary tumor and to which cancer cells metastasize from the primary tumor. Thus, the sentinel lymph node is known as a good index that indicates the metastasis of solid tumors to local lymph nodes. If there is no metastasis of cancer cells to the sentinel lymph node, it can be seen that there is no metastasis of cancer cells to other lymph nodes, suggesting that unnecessary dissection of lymph nodes is avoided. In other words, the biopsy of a sentinel lymph node in melanoma or breast cancer makes it possible to avoid unnecessary dissection of lymph nodes.
Indocyanine green has shortcomings of low hydrophilicity, low light stability, low photon yield, and low sensitivity. Further, indocyanine green has shortcomings in that it susceptible to nonspecific aggregation and is chemically degraded by external light, solvents and a change in temperature. In addition, it has a problem in that it is easily absorbed by serum proteins due to its low molecular weight and hydrophobic nature, and is eliminated through the kidneys. The blood half-life of indocyanine green is about 2-4 minutes. Thus, the development of an indocyanine green-based probe having a long blood half-life is required to increase the efficiency of diagnosis.
Furthermore, because indocyanine green easily diffuses in vivo, there are shortcomings in that a surgical operation for dissecting a sentinel lymph node using indocyanine green imaging should be performed within 30 minutes and in that a deep lymph node cannot be detected. In an attempt to overcome such shortcomings, nanomaterial-based indocyanine green probes, for example, nanoparticles, liposomes, micelles and the like, have been studied, and there have been studies focused on increasing the in vivo and in vitro stability of indocyanine green by encapsulating indocyanine green in nanoparticles or by using micelles composed of polymers, phospholipids and calcium phosphate (A. K Kirchherr et al. Mol Pharm. 6:480, 2009, V. B. Rodriguez et al. J Biomed Opt. 13:14, 2008: E. I. Altinoglu et al. Nano. 2:2075, 2008). When indocyanine green was encapsulated in nanoparticles, the physical and chemical stability of indocyanine green against external light and temperature were significantly increased, but when an indocyanine green probe was applied for clinical diagnosis, it was not specific for the site to be imaged.
Another typical probe for lymph node imaging is a lymph node imaging probe prepared by conjugating a radioisotope to mannosyl serum albumin, which is crucially used in the imaging and diagnosis of lymph nodes in the nuclear medicine field (G. Mariani, et al. J. Nucl. Med., 42:2001). Particularly, when radioactive colloids are injected into breast cancer or melanoma tumors and absorbed into lymph nodes, a sentinel lymph node, which is the first lymph node to which the colloids moves, can be imaged, dissected, and biopsied to determine whether cancer cells metastasized to the sentinel lymph node. Thus, there is an advantage in that unnecessary dissection of lymph nodes during surgical operations can be avoided. However, the probe having a radioactive isotope conjugated thereto is a probe that is used to determine the position and orientation of lymph nodes before a surgical operation, and has a shortcoming in that it cannot be used for real-time imaging during a surgical operation. Thus, it is required to develop a probe that can be used for real-time imaging during a surgical operation.
Under such technical circumstances, the present inventors have found that a complex of indocyanine green and mannosyl serum albumin overcomes the shortcoming of low in vivo stability of indocyanine green, and that when it is injected during a surgical operation, it can accurately detect the position of lymph nodes by detecting and imaging macrophages abundantly present in lymph nodes, thereby completing the present invention.